In modern society, over the past fifty years, there has been a shift from metals to polymers, partly because the latter do not oxidize in the environment. For example, corrosion of metal destroys valuable property. Corrosion is the inevitable reaction of metal to form metal oxide when the metal is exposed to environmental conditions that permit such corrosion. Billions of dollars are spent each year to repair or replace metallic structures because of such corrosion.
The corrosion of iron-containing articles, the natural process normally called “rusting”, has prompted considerable effort to find effective, economical ways to prevent or reduce rust. Coating the surface of the iron-containing article, the process of painting, was first attempted to shield the article from the elements needed for the natural rusting reaction to begin.
Iron-containing articles form the structures that have erected the cities and commercial links between them. Ironwork, for such diverse uses as multi-story buildings, suspension bridges, tunnels beneath a mountain or a river, high tension utility powerlines, fuel storage tanks, the Statue of Liberty, the Eiffel Tower, and reinforcement grids for concrete structures of all types, all require such protection from corrosion.
Anti-corrosion mechanisms have taken advantage of the Galvanic Series, whereby a less noble metal is a sacrificed in the environment where the iron would otherwise rust. This “cathodic protection” of metal has spawned an enormous industry dedicated to preserving metallic property against the ravages of the environment.
Cathodic protection utilizes the physics of a galvanic circuit, which can be assisted by power to be an active circuit to drive the corroding effects away from the metal being protected or which can be passive without power. Examples of passive galvanic circuitry are disclosed in U.S. Pat. No. 5,650,060 (Huang et al.) for an electrode-based system and in U.S. Pat. No. 5,976,419 (Hawkins et al.) for a coating-based system. Both types of systems rely on a more anodic metal in the Galvanic Series, such as zinc, to protect the more valuable iron in the structure. In the Huang et al. electrode, the zinc is in the form of plate adhered by an ionically conductive adhesive to a structure. In the Hawkins et al. coating, the zinc is in the form of particles dispersed in the binder and inherently conductive polymer. In both cases, the zinc is the anode of the galvanic circuit. The anodic zinc is sacrificed to preserve the cathodic iron.
U.S. Pat. No. 5,700,398 (Angelopoulos et al.) discloses a composition comprising polymeric matrix with a conductive filler comprising electrically conductive particles and a conducting polymer, such as a substituted polyaniline, wherein the amount of conducting polymer can be as low as 0.3 weight percent of the total polymeric matrix. But the electrically conductive particles are disclosed to be no less than about 40% by weight of the total polymeric matrix. Thus, by calculation, the electrically conductive particles are no less than about 28% by weight of the composition.
The combination of inherently conductive polymer with sacrificial metal particles becomes a cathodic protection coating to protect metal substrates in which the metal particles are less noble than the metal of the substrate. Such cathodic protection coating is disclosed in U.S. Pat. Nos. 6,440,332 and 6,627,117 (both Geer et al.). Such cathodic protection coating is marketed by PolyOne Corporation under the brand Catize® coating. As disclosed in U.S. Pat. No. 6,440,332, the amount of inherently conductive polymer is from approximately 1% to 40% by weight of the coating system and the amount of metal particles is from approximately 5% to 50% by weight of the coating system.
Japanese Patent Publication 8-277370 discloses a primer for general offshore structures, wherein the amount of the organic resin is 20 to 50 parts by weight, the amount of the metal particles (less electropositive than iron) is 60 to 90 parts by weight, and the amount of electroconductive polymer is 0.1 to 10 parts by weight.
The ingredients of a cathodic protection coating are expensive, especially the inherently conductive polymer.